Purification of both the stimulatory and inhibitory components of plasma acting on mouse spleen and marrow; testing of lipids extracted from these materials, the lipid-free protein, and the two together. Possible experimental simulation of clinical myelofibrosis by long period injections of the above material. Preparation of antibodies and study of their effects. Large scale preparation and purification of the spleen protein which stimulates erythroblast maturation in vitro. Preparation of antibodies. In vivo studies of the effect of antigen and antibody. Methods will be sought to separate stimulation of erythroblast generation from the subsequent maturation process. Such methods are needed for the study of the effects in vivo of the substances found to stimulate erythroblast maturation in vitro and also for the study of the question suggested by clinical and experimental evidence whether there are other humoral factors besides erythropoietin directly involved in normal erythropoiesis.